In 1996, a DVD (digital versatile disk) system having a recording capacity of 4.7 GB was developed by using an AlGaInP red semiconductor laser (a wavelength of approximately 650 nm). A conventional CD (compact disk) system had used an AlGaAs near infrared semiconductor laser (a wavelength of approximately 780 nm) and had a recording capacity of 650 MB.
There are many points differentiating the DVD system from the CD system. One of them is the base material thickness of an optical disk used. Specifically, the base material thickness of the optical disk is 1.2 mm in the CD system, while it is 0.6 mm in the DVD system. Accordingly, in order to obtain a compatibility with the CD system, various methods are being suggested in the DVD system.
One of them is a configuration using a bifocal lens (for example, see “Optical Review,” Vol. 1, No. 1, pages 27–29 (1997)).
The bifocal lens is a lens in which concentric circular hologram elements are formed on an objective lens with a numerical aperture NA of 0.6 designed for the 650 nm wavelength. This bifocal lens can separate light that is focused without aberration on a CD with a base material thickness of 1.2 mm by using a + first order light of the hologram element from light that is focused without aberration on a DVD with a base material thickness of 0.6 mm by using the usual objective lens (a zeroth-order light of the hologram element), thereby achieving the compatibility between the CD and the DVD.
However, although the optical system using the bifocal lens realizes the compatibility with the CD, it cannot yet achieve that with a CD-R. This is because a sufficient reproducing signal cannot be obtained due to the rather poor reflective properties of the CD-R in the red region. Thus, an optical pickup having two integrated units 125 and 126 (wavelengths of 650 nm and 780 nm) as shown in FIG. 21 is suggested.
In the configuration shown in FIG. 21, a laser beam with a wavelength of 650 nm emitted from the integrated unit 125 for DVD passes through a wavelength branching prism 127, a polarizing hologram 128 (diffraction gratings are formed on a LiNbO3 substrate by a proton exchange) and a wave plate 129 (a ( 5/4) λ plate for 650 nm wavelength), and then is focused on an optical disk (DVD-ROM) 131 by an objective lens 132. The light reflected by the optical disk 131 enters the wave plate 129 by which a polarization direction thereof is rotated by 90° from that of an incident light, is diffracted by the polarizing hologram 128 and is imaged on a photo detector (PD) in the integrated unit 125 for DVD. In this optical detection system, the focusing direction is controlled by a SSD (spot size detection) method, and the tracking direction is controlled by a phase difference detection method.
On the other hand, a laser beam with a wavelength of 780 nm emitted from the integrated unit 126 for CD passes through a plastic hologram element 126b with a narrow pitch and is reflected by the wavelength branching prism 127. Then, as the laser beam with a wavelength of 650 nm from the integrated unit 125 for DVD, it passes through the polarizing hologram 128 and the wave plate 129, and is focused on an optical disk (CD or CD-R) 130 by the objective lens 132. The light reflected by the optical disk 130 passes through the wave plate 129 and the polarizing hologram 128 again. In this case, since the wave plate 129 functions as a λ plate for 780 nm wavelength, the polarization direction is maintained and not diffracted by the polarizing hologram 128. The light reflected by the wavelength branching prism 127 and then diffracted by the plastic hologram element 126b is imaged on a photo detector (PD) in the integrated unit 126 for CD. In this optical detection system, the focusing direction is controlled by the SSD method, and the tracking direction is controlled by a three-beam method.
The objective lens 132 is designed so that the 780 nm wavelength light causes small aberration for the optical disk (CD or CD-R) 130 with a base material thickness of 1.2 mm and the 650 nm wavelength light causes small aberration for the optical disk (DVD-ROM) 131 with a base material thickness of 0.6 mm.
With the optical pickup having the above configuration, the optical disk (CD or CD-R) 130 with a base material thickness of 1.2 mm is reproduced by the laser beam with a wavelength of 780 nm emitted from the integrated unit 126 for CD, and the optical disk (DVD-ROM) 131 with a base material thickness of 0.6 mm is reproduced by the laser beam with a wavelength of 650 nm emitted from the integrated unit 125 for DVD, thereby obtaining excellent reproducing properties.
The recording capacity of a current DVD is 4.7 GB, and approximately two hours of NTSC (National Television System Commitee standard) broadcast data can be recorded. However, in order to develop media for image data of a high-vision or a high-definition (generally referred to as “HD” in the following), it is essential to further improve the recording density of the optical disks.
As means for improving the recording density of the optical disks, (1) changing a light source so as to produce a shorter wavelength and (2) increasing the numerical aperture NA of the objective lens can be considered. However, making the numerical aperture NA of the objective lens larger than the current value of 0.6 is difficult both from the viewpoints of margins on systems and of compatibility with the CDs and the DVDs.
On the other hand, the light source can be changed so as to produce a shorter wavelength by using a second harmonic generation (SHG) technique of a near infrared semiconductor laser, or by using a GaN semiconductor laser. The use of blue light with a wavelength of approximately 400 nm can improve the recording density by approximately 2.3 times over the current DVD. In the following, the DVD that is obtained as above will be referred to as “HD-DVD”.
Also in the era of HD-DVDs using blue light, it is important to obtain compatibility with the DVDs and the CDs. In line with the CD-R, a pigment-type DVD-R is currently being developed. However, the reflective properties of the CD-Rs and the DVD-Rs deteriorate in the blue region. Therefore, in order to achieve compatibility, an optical pickup that is provided with coherent light sources respectively emitting lights in three wavelength regions of blue region, red region and infrared region is necessary.
However, since the optical pickup that is configured with the coherent light sources of multiple wavelengths necessitates many optical components, it is difficult to design an optical pickup that can be mass-produced on a practical level. For example, problems described below are caused.
(1) Due to the increase of the number of the optical components, higher precision in aberration of respective optical components becomes necessary, and integration thereof becomes difficult, leading to the difficulty in designing a small-size (thin) optical pickup, and
(2) Since it is necessary to simultaneously achieve the optical detection systems corresponding to respective lights of multiple wavelengths, the configuration of a ¼ wave plate becomes complex when using a polarizing hologram element and a polarization branching element in the optical detection systems.